Aircraft have to be navigated during flight. Manned aircraft are generally navigated by a pilot in command on board of the aircraft. Unmanned aircraft such as remotely piloted aircraft (RPA) are generally navigated automatically using on-board piloting computers. In some systems, high-level commands may be submitted by an off-board pilot being located, for example, in a ground control center and remotely piloting the aircraft using communication channels. Sometimes, RPAs are also referred to as unmanned aerial vehicles (UAV) or unmanned aerial systems (UAS).
In order to be able to follow a desired air route, navigation needs to be based on precise information about the present position of the aircraft. Typically, such position information should be three-dimensional (3D).
In modern aircraft, position information may be acquired using a GPS-receiver. Such a GPS-receiver is adapted to receive GPS-signals emitted by GPS-satellites and, based on such GPS-signals, determine position information for the aircraft.
Generally, GPS-satellites emit two types of GPS-signals. A first type is referred to herein as C/A-GPS-signals. Such signals are sometimes also referred to as course/acquisition code or C/A code. Such C/A-GPS-signals allow for a determination of a position information with sufficient precision for use in civil aircraft navigation. A second type of signal is referred to herein as P(Y)-GPS-signals. Such signals are also referred to as precision code or encrypted precision code. Generally, such P(Y)-GPS-signals allow for aircraft navigation with higher navigation precision than C/A-GPS-signals. Furthermore, as these P(Y)-GPS-signals are mainly intended for military navigation use, these signals are encrypted in order to minimize a risk of spoofing.
Generally, only military-type GPS-receivers are allowed to be used for determining position information based on P(Y)-GPS-signals. The technology of such military-type GPS-receivers is to be kept secret. Generally, military-type GPS-receivers are allowed to be used in “state aircraft” only, i.e., in aircraft for military applications, police applications, emergency management applications, and similar, such “state aircraft” typically being under governmental control.
On the other hand, civil aircraft are only allowed to be navigated using civil-certified GPS-receivers. Such civil-certified GPS-receivers are used for determining a position information based on C/A-GPS-signals. Each GPS-receiver used in civil aviation generally has to be certified by a governmental agency such as, e.g., the Federal Aviation Agency (FAA).
Unintentional or intentional GPS-signal-in-space degradation, sometimes referred to as “jamming,” and intentional GPS-signal-in-space alteration by third parties, sometimes referred to as “spoofing,” may pose a great threat to civil or military aviation since the aircraft position as measured by on-board civil or military GPS-receivers may be impaired, in the case of jamming, or deliberately altered, in the case of spoofing, by third parties on-ground and in-air. This may lead to degradation or loss of the aircraft GPS position and, in worst-case scenarios, even to deliberate aircraft hijacking.
Furthermore, communication channels may be jammed or spoofed. This may be especially problematic for remotely piloted aircraft since the pilot can no longer actively control the RPA. Communication channels can be protected against spoofing with encryption, but it is very hard to protect them against jamming.
U.S. Pat. No. 6,377,892 B1, US 2005/0146459 A1 and U.S. Pat. No. 6,577,951 B1 address aircraft navigation using different types of GPS-receivers.